1. Field of the Invention
The present invention is directed to a process and apparatus for sputtering a surface of a workpiece to be sputter-etched or for sputtering a surface of a target for sputter-coating a workpiece, both referred to as a "sputtering object".
More specifically, the present invention is directed to such processes and apparatus, whereat RF sputtering is performed in a vacuum recipient which is filled with a working gas at a selected gas pressure.
Even more specifically, the present invention is directed to the application of magnetic fields in such processes and apparatus.
2. Description of Prior Art
In specific art mentioned above, the law of KOENIG as disclosed for instance in H. R. Koenig and L. I. Meissel, IBM Journal Research Development 14, p. 168 (1970), and H. R. Koenig, U.S. Pat. No. 3,661,761 (1969), is well-known. It defines that the ratio of drop of time-averaged electric potential adjacent to electrode surfaces between which an RF plasma discharge is generated, is given by the inverse ratio of respective electrode surface areas raised to the fourth power. This law is only valid under specific conditions:
The discharge space of the RF plasma discharge is confined by only two electrode surfaces between which RF energy is applied. No further electrode surface is exposed to the plasma which is loaded with an RF current. The confinement of the RF plasma discharge space by the two electrode surfaces whereat RF energy is applied may only have gaps or holes which are of such small extent that the plasma discharge may not spread out of the confinement and couple RF currents to other parts of a vacuum chamber. This means e.g. that the minimal diameter of any gaps in such a two electrode confinement must substantially be not larger than the dark space distance at the working gas pressure maintained during RF plasma discharge. Further spacings between the two electrodes may e.g. be bridged by dielectric material which also prevents spreading of the discharge.
If a sputtering object or a workpiece to be sputter-coated is disposed within the confined discharge space, the said condition is further fulfilled only if the sputtering object is either electrically floating or is disposed on the electric potential of one of the two electrode surfaces to which RF energy is applied.
If these conditions are taken in consideration and RF energy is applied at a frequency of above 3 MHz and below about 90 MHz, then the KOENIG law mentioned above will at least approximately be fulfilled.
In a strongly simplified consideration, positive ions out of the RF plasma discharge are accelerated to the respective electrode surfaces at a kinetic energy predominantly given by the drop of time-averaged electric potential adjacent the electrode surface considered. Depending on the material to be sputtered and the kind of ions and thus of the working gas, sputtering starts at a given ion accelerating drop of time-averaged electric potential adjacent an electrode surface considered.
The "sputtering rate" defined as mass of material sputtered off a surface per time unit depends predominantly upon two largely independent entities:
a) the average kinetic energy of the positive ions, given by the said drop of time-averaged electric potential across a dark space region, PA1 b) "plasma density" in such dark space region given by the density of electrically charged particles in said space. PA1 providing in a vacuum recipient a first and a second electrode with a first and a second electrode surface respectively; PA1 selecting a gas pressure for a working gas to be applied to said vacuum recipient in a region defined; PA1 confining a discharge space in said recipient by said first and second electrode surfaces, thereby preventing an RF discharge generated between said first and second electrode surfaces to spread outside said confinement; PA1 generating an RF plasma discharge in said discharge space with said working gas at said selected gas pressure by applying an electric RF field between said first and second electrode surfaces, thereby generating in said space and adjacent said first electrode surface a first dark space region with a first drop of time-averaged electric potential and adjacent said second electrode surface a second dark space region with a second drop of time-averaged electric potential, said first and second drops of time-averaged electric potential falling towards said first and second electrode surfaces respectively; PA1 selecting the ratio R.sub.A12 of the areas of said first and second electrode surfaces to be EQU 0,.3.ltoreq.R.sub.A12 &lt;1; PA1 disposing said surface of said object in said second dark space region adjacent said second electrode surface being larger than said first electrode surface so as to face said first dark space region, thereby disposing said surface of said object one of at a floating electric potential and at the electric potential of said second electrode surface, so as to perform diode sputtering; PA1 enabling sputtering of said surface of said object adjacent said second and larger electrode surface by applying a magnetic field within said discharge space, a predominant part of its lines of force being tunnel-like shaped on said first electrode surface and across said first dark space region. PA1 providing in a vacuum recipient a first and a second electrode with a first and a second electrode surface respectively; PA1 selecting a gas pressure for a working gas to be applied to said vacuum recipient; PA1 confining a discharge space in said recipient defined by said first and second electrode surfaces, thereby preventing an RF discharge generated between said first and second electrode surfaces to spread outside said confinement; PA1 generating an RF plasma discharge in said discharge space with said working gas at said selected gas pressure by applying an electric RF field between said first and second electrode surfaces, thereby generating in said space and adjacent said first electrode surface a first dark space region with a first drop of time-averaged electric potential and adjacent said second electrode surface a second dark space region with a second drop of time-averaged electric potential, said first and second drops of time-averaged electric potential falling towards said first and second electrode surfaces respectively; PA1 selecting the ratio R.sub.A12 of the areas of said first and second electrode surfaces to be 1.ltoreq.R.sub.A12.ltoreq.3; PA1 disposing said surface of said object in said second dark space region adjacent said second electrode surface being smaller than said first electrode surface so as to face said first dark space region, thereby disposing said surface of said object at one of a floating electric potential and the electric potential of said second electrode surface, so as to perform diode sputtering; PA1 reducing said first drop of time-averaged electric potential falling towards said first electrode surface being equal or larger than said second electrode surface below a value which would lead to substantial sputtering of said first electrode surface by applying a magnetic field within said discharge space, a predominant part of its lines of force being tunnel-like shaped on said first electrode surface.
The sputtering rate may be increased by increasing the average kinetic energy of ions and/or by increasing the number of ions impinging on the surface to be sputtered. Thereby increasing of the plasma density will only then increase the sputter rate if the average energy of the ions suffices for sputtering at all.
The law of KOENIG only considers the ratio of averaged kinetic energies in relation to ratio of electrode surfaces at homogenous plasma density.
It thus becomes evident that according to the law of KOENIG, when the two electrode surfaces are equal, both these electrode surfaces will be subjected to sputtering at equal kinetic energy, because in the adjacent dark spaces of both electrodes equal drops of time-averaged electric potential will occur. If one of the two electrodes is made smaller than the other, this will result in an increased ion accelerating drop of time-averaged electric potential adjacent the smaller electrode surface and across its dark space and, accordingly, to diminution of such ion accelerating drop adjacent the larger electrode surface and across its dark space region.
As was mentioned, this phenomenon is known to prevail if the conditions mentioned above are considered.
From a DC plasma sputtering technique, wherein an electric DC field is applied between two electrode surfaces, it is known to provide on one of the two electrode surfaces to be sputtered, here clearly the cathode, a tunnel-shaped magnetic field to improve plasma density adjacent the cathode by the well-known electron trapping effect of magnetic force lines aligned perpendicularly to the electric force lines.
Several successful approaches have become known to apply an e.g. so-called magnetron technique, known from DC sputtering technique, also to RF sputtering techniques, with the object of, as in the DC sputtering case, improving the sputter rate by rising plasma density adjacent the surface to be sputtered. For simultaneously improving sputtering homogenity along a surface to be sputtered, it further became known to provide a relative movement between an applied magnetic field pattern and the surface to be sputtered.
The present invention, as will be described below, is based on a new recognition made by the inventors at systems for which the law of KOENIG is principally valid and which, thus, fulfil the above mentioned conditions by inventively applying specific magnetic fields: It becomes possible to realize average kinetic energy of the ions impinging upon the electrode surfaces which are in opposition to those predicted by the KOENIG law. This inventively recognized deviation of the distribution of the said energy at the two electrode surfaces from that predicted by KOENIG is especially pronounced at electrode surfaces which are of the same order of extent.
From the U.S. Pat. No. 4,278,528 patent (Kuehnle) it is known to provide in an extended vacuum chamber a multitude of targets to be sputtered by RF plasma discharge. Between the multitude of targets and a workpiece band to be continuously sputter-coated a mask in a form of a metallic and grounded plate with respective slits is provided, and provides for sputter-coating a specific line pattern on the moving workpiece band. The RF plasma discharge spaces are formed between respective targets and "anode" electrodes, whereby the RF plasma may spread laterally outwards along the surfaces of the targets. This is because the targets and "anodes" do not confine the respective discharge spaces laterally. Thus, the plasma discharge spaces are primarily confined or bordered by the overall vacuum chamber wall, targets and counter-electrodes named "anodes". Tunnel-shaped magnetic fields are applied either on the target or, opposite to the target, to the "anode" electrode surfaces, so as to prevent electrons from heating the workpiece band which may consist of paper or plastic material.
If this arrangement is considered under the law of KOENIG, then it must be considered that the discharge space is confined on one hand by the target electrode surfaces and, on the other hand, by the "anode"electrode surfaces plus all metallic surfaces exposed to the inside of the overall vacuum chamber. As was mentioned above, the law of KOENIG is further only valid if the discharge is bordered by surfaces on only the two electrode potentials respectively externally applied and on no third potential externally applied and the discharge is thus generated in a so-called "diode arrangement".
In the above mentioned U.S. Pat. No. 4,278,528 patent the diode arrangement condition is only fulfilled if the wall of the overall chamber is at the same electric potential as the biased counter-electrodes (so-called "anodes").
If under these conditions--confinement and diode operation--this known arrangement is considered under the law of KOENIG, it is evident that the electrode surfaces formed by all metallic surfaces exposed to the inside of the vacuum chamber must be considered as one electrode surface and are extremely larger than the "cathode" surfaces of the targets. Thus, according to the said KOENIG law, the "cathode" surfaces will be exclusively sputtered and all the counter-electrode surfaces will not be sputtered, because, there, kinetic ion energy is not sufficient. There is a big difference in the surface areas of the surfaces to be considered as electrode surfaces. The sputtering/non-sputtering energy distribution is thus purely governed by the law of KOENIG and the magnetic fields applied will not affect this distribution. These magnetic fields may increase the plasma density at the targets and thereby sputter rate, i.e. at that electrode at which the average ion energy suffices for sputtering anyhow.
U.S. Pat. No. 4,572,759 patent (Benzing) discloses an RF sputtering arrangement which comprises a pair of coaxial cylindrical electrodes. Electrically isolated from the outer cylindrical electrode, further a wafer carrier electrode is provided. For sputter-etching a wafer on the third electrode, electrical RF potential is applied between the wafer carrier electrode and the central cylindrical electrode, and the second electrode, formed by the outer cylinder, which is disposed at ground potential. There is, thus, formed a triode sputtering arrangement, in that the RF plasma discharge is confined by two electrodes mutually disposed on RF potential and one further electrode disposed at ground potential is also loaded by RF current. In such a triode arrangement, the law of KOENIG may not be applied, as the distribution of the energy of ions impinging on the three electrodes is largely influenced by potentials applied to the three electrodes.
U.S. Pat. No. 4,657,619 patent (O'Donnell) discloses a sputtering apparatus which may be operated in a diode operating mode: For sputter-etching a workpiece, it is disposed on a workpiece carrier electrode within a vacuum recipient and fed with RF energy. The metallic wall of the vacuum chamber is apparently ground potential so that this arrangement in this possible operating mode acts as diode RF sputtering apparatus.
As the entire inner surface of the wall of the vacuum chamber acts as one of the two electrodes confining the RF discharge space and defines an electrode surface area which is much larger than the surface of the workpiece carrier electrode, application of the law of KOENIG reveals that it is, in fact, the work-piece carrier electrode which will be practically exclusively sputtered by ions with sufficient average kinetic energy, whereas the wall of the vacuum chamber will be sputtered significantly less, because said energy is too small.
The magnetic fields which are further applied in tunnel-shaped form across the workpiece carrier electrode and/or across the vacuum chamber wall electrode are designed to obtain a uniform plasma processing along the workpiece carrier electrode to thereby maximize the workpiece size that can be handled by given plasma processing apparatus. As the electrode surfaces, which do confine the RF plasma discharge space, are of highly different extents, which prevents sufficient kinetic ion energy to occur at the larger electrode surface, the magnetic fields applied do not affect the distribution of ion energy at one electrode relative to that energy at the other electrode, but the plasma density at the smaller electrode, where average kinetic ion energy suffices for sputtering.
A similar apparatus as disclosed in U.S. Pat. No. 4,657,619 patent is also disclosed in U.S. Pat. No. 4,581,118 patent (Class) for sputter-coating a workpiece. Thereby, opposite to the workpiece carrier electrode and electrically isolated from the chamber wall electrode, a target electrode is mounted and RF energy is mutually fed between target electrode and workpiece carrier electrode. Thereby, obviously and as customary, for instance in view of safety considerations, the wall of the vacuum chamber confining as a third electrode together with the target electrode and the workpiece carrier electrode the RF discharge space is disposed at ground potential. Thus, this apparatus is a typical triode sputtering apparatus.
The U.S. Pat. No. 4,632,719 patent (Chow) discloses an apparatus for sputter-etching a substrate which is disposed centrally in a vacuum chamber on a workpiece carrier electrode to which RF energy is fed. Integrated in and at the same electrical potential as the wall surrounding the vacuum chamber, there is provided a catcher plate disposed opposite to the workpiece carrier electrode. Further, the workpiece carrier electrode is surrounded by a shield ring disposed at ground potential as is the wall of the vacuum chamber and thus the catcher plate.
In this apparatus, which acts as a typical diode sputtering apparatus, the RF plasma discharge is confined on one hand by the workpiece carrier electrode and, on the other hand, by the shield ring, the catcher plate and significant parts of the vacuum chamber wall surrounding the catcher plate. Applying the law of KOENIG to this arrangement reveals that due to the large ratio of electrode surface areas confining the RF discharge space, the average kinetic energy of ions will only suffice for considerable sputtering at the workpiece carrier electrode. A tunnel-shaped magnetic field is applied to the catcher plate and chamber wall electrode. This tunnel-shaped magnetic field generates a magnetic field in close proximity to the face of the wafer to be etched on the workpiece carrier electrode. Thereby, there, plasma density is improved, resulting in an improved etching rate and an improved uniformity of etching of the wafer. Thereby, a simple and inexpensive construction is realized due to mounting of the magnets to the grounded catcher plate for producing magnetic fields in close proximity to the face of the wafer to be etched.
Thus, in all diode sputtering apparatus mentioned above, the KOENIG law is considered in that the electrode to be sputtered is constructionally made significantly smaller than the second electrode confining the RF discharge. Thereby the applied large ratio of the two electrode surfaces confining the RF discharge space leads to a drop of time-averaged electric potential in the dark space region adjacent the small electrode considerably larger than such drop across the dark space region adjacent to the much larger electrode. Thereby, ion acceleration at the latter is, averaged, insufficient to lead to significant surface sputtering.
Any magnetic field applied to a dark space region, where ion acceleration does not suffice for sputtering, will have no effect on such sputtering. The influence on the distribution of electric potential drops can be neglected because the potential drop at the large electrode is already very small compared with the potential drop at the small electrode. Magnetic fields applied to a dark space region, where ion acceleration suffices for significant sputtering, will improve the sputter rate by increasing the plasma density.
A possibility to increase the surface of an electrode is described in the U.S. Pat. No. 3,661,761 patent and consists of placing protuberances on the electrode in the vacuum chamber which is not to be sputtered, in order to enlarge its surface area.
The same approach of large electrode surface area is used in the U.S. Pat. No. 3,369,991 patent.
In all diode sputtering arrangements mentioned above, and as was mentioned, concentration of sputtering action to one of the two electrodes confining the RF plasma discharge space is realized by constructionally providing a large surface area ratio of large surface not to be sputtered to small surface to be sputtered. Thereby a significant drawback of such technique is that the overall extent of the discharge space and, accordingly of a respective vacuum chamber, is predominantly given by the large extent necessary of the large electrode confining the RF plasma space. This large electrode not to be sputtered is thus not exploited as a target surface to be sputtered or to deposit workpieces to be etched.
Thus, this large electrode makes the arrangement bulky and does, in fact, not contribute to the extent of surface to be sputtered, which is considerably smaller. This is due to considerations according to the said law of KOENIG which apparently was believed to be unavoidable.